So you're installing fresh? If so, when you run ./nodebb setup, it should ask you for DB info, URL, and admin username and password. The only case where it doesn't is if config.jsonalready exists. In that case, it should run everything, and generate an admin username and password which it prints out after everything else is done, including the build.

The service type can be simple so long as you execute loader.js with --no-daemon. If you want to use forking you can, you can then omit --no-daemon. Logging is tougher then, as it wouldn't be managed by systemd.